This invention relates to the design and construction of piezocomposite ultrasound arrays in conjunction with integrated circuits, and in particular to improvements in thermal and crosstalk performance in piezocomposite ultrasound array and integrated circuit assemblies.
Diagnostic ultrasound is an established and growing medical imaging modality. The configuration of a typical system includes a handheld probe connected to a host computer and image display unit by an umbilical cable carrying power, control signals and image data. Currently handheld probes using one-dimensional acoustical transducer arrays with up to 128 elements are the standard in the industry, although partial and full two dimensional arrays are well into development with several vendors.
Medical ultrasound systems transmit a short pulse of ultrasound and receive echoes from structures within the body. The handheld probes are most often applied to the skin using a coupling gel. Specialty probes are available for endocavity, endoluminal and intraoperative scanning. Almost all systems on the market today produce real-time, grayscale, B-scan images. Many systems include colorflow imaging.
Improved image quality requires the use of matrix (nxc3x97m) arrays with a thousand or more elements. As element numbers increase and their dimensions grow smaller, limitations to present fabrication technologies arise. Cost, ergonomics, produceability and reliability are important issues. Connecting an integrated circuit directly to the array elements alleviates these problems.
All linear arrays currently on the market use piezoelectric materials as the transducing mechanism from electrical signals to ultrasound (transmitter) and ultrasound back to electrical signals (receiver). The signals are generally in the form of short pulses or tone bursts.
Referring to prior art FIG. 1, most high performance acoustical arrays use a piezocomposite material, which is fabricated by a xe2x80x9cdice and fillxe2x80x9d technique as is suggested, for example, in Smith et al""s U.S. Pat. No. 5,744,898. The piezocomposite transducer array structure 9, having a planar array of piezocomposite transducer elements 10, provides improved bandwidth and efficiency as well as reduced crosstalk or interference between adjacent elements 10, relative to older designs.
Pieces of native ceramic such as Type 3203HD made by CTS Piezoelectric Products, or PZT-5H made by Morgan-Matroc, are diamond sawed in a crosscut pattern to yield pillars 18, there being multiple pillars in each piezocomposite element 10. The intra-element spaces 28 between the pillars are filled with a polymer, such as DER332 epoxy made by Dow Chemical. The inter-element spaces 24 are often left air-filled or are filled with a sound-absorbing polymer. Top electrode 30 is the common electrical connection between elements 10. Bottom electrodes 22 are delineated for each array element 10 and are used as the electrical connections to the piezocomposite material.
In a beamsteered transducer array, the dimensions of elements 10 are typically less than a wavelength (in water or tissue) in the steering dimension. For example, in a 3.5 MHz (1xc3x97128) array the element width is between 0.2 to 0.4 mm with a center to center spacing of 0.5 mm for a total array length of approximately 64 mm. In the other dimension, the element dimensions are a tradeoff between resolution and depth of focus. For a 3.5 MHz array, this dimension is typically 12 to 15 mm.
As the frequency of the array increases, element size decreases, as does element thickness, however, the aspect ratio remains constant. Other methods of fabrication such as laser milling or scribing, etching or deposition are under development. At present, they are not well accepted.
As is more fully described in the parent applications, in a fully assembled transducer scanner head, there is a backing behind the array and its supporting ASIC that provides mechanical support and acoustical attenuation. When a piezoelectric transducer is electrically pulsed, two acoustical pulses are generated that travel in opposite directions. The pulse traveling out of the scanhead is desired, while the pulse propagating into the backing is unwanted and is absorbed by the backing.
One or more matching layers are placed in the scanner head in the path of the desired pulse, to improve the coupling of energy from the piezocomposite into the body of the subject by matching the higher acoustical impedance piezocomposite to the lower acoustical impedance of the body. This matching layer functions in the same way as the anti-reflection coating on an optical lens.
The system electronics focus the pulse in the scanning plane dimension. A simple convex lens forms the front surface of the scanner head that contacts the patient""s skin. It provides a fixed focus to the sound pulse in the xe2x80x9cout-of planexe2x80x9d dimension, which is perpendicular to the scanning plane.
Modern systems impose increasingly stringent requirements on the acoustical arrays. Parameters that characterize typical medical ultrasound arrays are described in more detail in the parent applications, but with regard to this disclosure include in particular; crosstalk. Crosstalk is the interference of signals between array elements 10. The interference may be electrical, mechanical or acoustical. It is expressed in dB relative to the nearest neighbor element. Crosstalk in a well-constructed array is better than xe2x88x9230 dB.
Extension of the several inherent technologies to matrix transducer arrays is underway at most transducer and system manufacturers. As the number of elements increases and their size decreases, however, the existing approaches may no longer be feasible or practical. Processing time, touch labor, yield, reliability and cost become limiting issues and new processes are required.
In FIG. 2, a cross-section of a prior art piezocomposite integrated array, the array elements 10 are electrically and mechanically connected to integrated circuit (IC) substrate 32 with electrically conductive bumps 34 using metallized pads 36 on IC 32 to form a complete electrical circuit. Integrated circuit substrate 32 is typically composed of silicon, although other semiconductors may be used. Conductive bumps 34 may be composed of solder or a conductive polymer such as silver epoxy.
Placing an integrated circuit (IC) directly behind an ultrasound array is a well-known solution to the problem of many long cables connecting the array elements in a scanhead to electronics in a separate electronics console. The preferred method places an IC with unit cells of similar dimensions to the array elements 10 immediately behind the array elements. The corresponding elements and unit cells are bonded electrically and mechanically to the IC using micro-solder balls. The space between the IC and the array can be filled with a material such as epoxy for improved mechanical strength. However, this leads to excessive crosstalk (signal interference) between the array elements. Crosstalk leads to poor dynamic range and loss of contrast in the images.
One solution to this problem is simply to leave an air space between the array and IC. The air gap effectively prevents sound transmission into the IC. By limiting the contact area to the micro solder balls alone, which area is much smaller than the wavelength of sounds, crosstalk is effectively eliminated. Use of an air gap, however, results in a relatively narrow bandwidth transducer array, typically with 30 to 60% fractional bandwidth. Modern medical ultrasound systems require bandwidths of 100 to 120%.
In a United States government agency funded program, a real-time, 3D, ultrasound camera intended for Army medic use on the front lines was designed and its feasibility proven. In this camera, an acoustical lens was used to image a volume onto a 128xc3x97128 (16,384 element), 5 MHz matrix array. Each element of the piezocomposite array had a custom integrated circuit bump-bonded directly behind it using micro-solder balls. The piezocomposite array was air-backed, i.e. there was a small air space between the array and the IC. The inter-element spaces were filled with polymer. The bump bonds were the only mechanical and electrical connections between the array and the IC. No matching layer was used on the front side of the array.
Each unit cell of the ROIC contained a preamplifier, signal processing, a limited amount of sampled data storage and multiplexing. The silicon was two side-buttable, permitting tiling of four pieces into the square 128xc3x97128 array. This integrated matrix array had several important limitations: It operated as a receiver only. The array bandwidth was only 35%, which is too low for use in most other ultrasound systems, although it was adequate for the camera, in which the multiplexing is significantly different from that used by beamformed B-scan ultrasound systems. Array crosstalk was marginally acceptable and reliability was poor. These problems were traced to differential thermal expansion between the piezocomposite array and the IC.
Referring to FIG. 2, the thermal expansion of the piezocomposite structure 9 in the plane of the array is determined by the combination of thermal expansion of the ceramic of pillars 18 and the epoxy in spaces 24 and 28. For example, PZT-5H, a typical piezo ceramic has a coefficient of thermal expansion (CTE) of 4xc3x9710xe2x88x926 per degree C., whereas DER 332, a typical epoxy, has a CTE of 40xc3x9710xe2x88x926 per degree C. or ten times that of the ceramic. Thus, the thermal expansion of a piezocomposite in the plane of the array is dominated by the epoxy. The CTE of silicon is about 7xc3x9710xe2x88x926 per degree C., which is a factor of five smaller than such a piezocomposite.
Temperature changes cause piezocomposite array 9 to expand or contract at a different rate than integrated circuit 32. Bump bonds 34 are stressed by these differential expansions. This stress may cause the bumps to rip apart. The adhesion at the interface between the bumps 34 and the array element electrodes 22 or at the integrated circuit pads 36 may also fail. One or more of the array elements 10 then becomes electrically and mechanically separated from the integrated circuit and the array becomes impaired or non-functional. As the size of the array increases to larger numbers of elements, the effect of the differential thermal expansion becomes worse. A five degree C. temperature change may be enough to cause failure. In the extreme, the array may even separate from the integrated circuit completely.
Clearly, there is a need for improved thermal performance while maintaining or improving acoustical isolation to reduce crosstalk and related interference between array elements.
It is among the goals of the invention to provide a piezocomposite transducer array and integrated circuit (IC) assembly that has better lateral acoustical isolation between transducer elements. It is another goal to provide a transducer array and IC assembly with improved mechanically reliability at the solder bond join between the array and the IC. It is also a goal to provide for greater vertical attenuation of acoustical signals as between the array and the IC.
It is an additional goal to provide a common electrode for the distal end of the elements of the array that combines acoustical matching properties suitable to the frequency of the array and the medium to which it is being applied.
To these ends, the invention includes a piezocomposite ultrasound array with approximately matched thermal expansion characteristics to silicon or other semiconductor material from which IC""s are commonly constructed. The invention enables present fabrication techniques to be extended to larger and multi-dimensional arrays while maintaining the desirable properties of piezocomposite arrays such as high sensitivity, low crosstalk and relatively easy fabrication.
One aspect of the invention is the combination of a supporting substrate of the same thermal expansion characteristics as the IC, attached directly to the base end of the transducer elements so as to hold them in thermal alignment with the IC which is bump bonded with micro solder balls to the underside of the supporting substrate, thereby limiting the mechanical shear stress on the bump bonds caused by differential thermal expansion. The material in the spaces between the elements is selected to absorb the excess thermal expansion of the individual piezocomposite elements, so that the array as a whole can be constrained to the desired rate of expansion.
Another aspect of the invention provides for combining the elements of the first aspect with a common electrode layer at the distal end of the array elements that is configured to be an acoustical matching layer between the array the medium to which it is being applied.
Yet another aspect of the invention is the use of a thinned integrated circuit, or multiple thinned layers of integrated circuit, in an integrated ultrasound transducer array, which thus becomes essentially acoustically transparent. This technique minimizes crosstalk transmission through the IC and enables wide bandwidth transducers. Acoustical transparency requires the thickness of the IC to be selected to produce a minimum reflection at or near the center or midrange frequency of the transducer array. A thinned IC also permits the use of conventional ultrasound transducer designs including acoustically attenuating backing structures bonded to the IC, which are well known in the ultrasonic transducer art.
Other and additional goals and advantages of the invention will be apparent to those skilled in the art upon review of the abstract, description, figures and claims that follow.